1. Field of the Invention
This invention relates to a non-volatile semiconductor memory device and an electric device with the same.
2. Description of Related Art
Currently known EEPROMs are formed to have a type of memory cell which stores charge in a floating gate thereof. A NAND type flash memory, which is known as one of these EEPROMs, has a cell array in which NAND cell units are arranged. Each NAND cell unit is configured to have a plurality of memory cells connected in series. In the NAND cell unit, sources and drains are shared between adjacent memory cells. Therefore, NAND type flash memory has a property that it is easy to increase the memory capacity with a relatively small chip size by increasing the number of cells in the NAND cell unit.
A NAND cell unit is, as described above, configured to have a plurality of memory cells connected in series, and connected to a bit line. A data read operation is done by detecting whether the bit line is discharged or not by a selected cell. Applied to each unselected cell in the NAND cell unit is a pass voltage which is able to turn on it without regard to data held therein. However, due to the fact that plural cells are serially connected in the NAND cell unit, the cannel resistance of the NAND cell unit is large, and cell current is small at the data read time. Conventionally, as a sense amplifier scheme for data determining based on the cell current, a method of judging a voltage difference due to the cell current difference by use of an inverter threshold voltage (refer to, for example, Published and Unexamined Japanese Patent Application (kokai) No. 11-260076).
However, the conventional sense amp scheme for data determining based on the inverter threshold has some problems in viewpoints of lowering the power consumption and increasing the memory capacity in future. As the number of memory cells in a NAND cell unit is increased more, or the power supply voltage becomes lower, the sense margin becomes less. As a result, it becomes difficult to perform a high speed data read operation. Alternatively, there is a fear that data determination becomes impossible.